On: Cortana available on iOS and Android

[arihant]

I think both Siri and Google Now offer third-party hooks, even on the watches. I'm not sure about Cortana.

[MBCook → arihant]

Siri does not, at least at the moment. It's widely hoped that it will, but today no.

[mikeash → MBCook]

A notable exception to this is HomeKit devices. While it's not close to what you'd want for a general third-party API, it does let you integrate some things. I've messed around a bit with using a server to fake being a HomeKit device to get some custom things working with Siri.